This will be my first year taking my DSLR to the slopes, I want to get the best pics possible.

I have thought for a long time that the main thing I would need is a circular polariser to eliminate some of the glare from the snow but after watching a youtube video of a guy doing snow photography he suggested that it will make the sky look really dark because of something to do with altitude.

Slightly off topic but related - when taking pictures with snowy background, your camera is going to want to dial-down the exposure too far because the amount of incoming light is higher than normal. You'll want to make sure you compensate by turning UP your exposure to where your light meter says you're over-exposing by 2/3 of a stop to a full stop
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StormeHawkeFeb 13 '14 at 17:59

Even more, in bright sun even +2 often is a good value.
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hisFeb 28 '14 at 19:58

Personally, I'd try it and see which works better. If the polarization kills too much of the light from the sky, then I'd remove it but if there is too much glare from the snow, I'd leave it. Both could be problems and the amount of each problem is going to depend on the composition and angle of your shots. If you don't have much sky, then the sky being a darker blue doesn't matter and if you are shooting with the sun to your back, there isn't going to be a whole lot of glare off the snow.

The main benefit I see is that by using a polariser I will get more punchier colours while using exposure comp to adjust the white of the snow. The drawbacks is that I am not totally sure if I want to keep adjusting the polarizer when moving between landscape or portrait and can see it taking long if I just want a quick snap straight out of my bag. That's why I am asking if in general they are a must for winter sports, because I don't want o be unprepared.
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connerszFeb 13 '14 at 14:35

"Punchier colors" is a not a good reason for a polariser in a digital workflow. A polariser has still its benefits, most other filters are better to be simulated later. But if the pure color effect of the polariser is your main reason for this filter, then the answer to your question is a clear "no, don't use it".
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hisFeb 13 '14 at 22:44

This question really comes down to what, if anything, in winter sport scenes will be polarized, and then eliminating some polarity will make the shot better?

The first obvious answer is the sky if it's clear blue. The deep blue is not polarized, but the whiter haze in front of that is, depending on angle to the sun. By adjusting the polarizer to minimize the haze, you make the sky appear deeper and darker blue. Most of the time I think that makes for a better picture, but that is subjective and up to you and what you want to show.

Reflections off of ice at certain angles will also be polarized. A polarizer can be used to either reduce or accentuate these reflections. Again, whether that's good or bad and which way is up to you and what you are trying to show.

The diffuse white light from snow will have some polarization at certain angles from the sun. This won't be as dramatic as reflections off of ice, but could sometimes be used to advantage to allow the texture of the snow to be seen better and making it appear less blown out.

So yes, a polarizer can be useful for some types of shots in some conditions. Whether that matters or whether that produces a desirable effect is only something you can answer.

Overall, when shooting with snow around, be careful not to overexpose it. You might want to set the automatic exposure to -1 or so, or set it to take many points into account, not just the center. All the snow blown out is a lot more crappy looking than the little bit a polarizer will add back - unless of course that's what you are trying to do for some reason.

The automatic exposure will lead to under exposure because the snow is significantly brighter than 18% grey but the automatic will treat it as such; so you should set the exposure compensation to + 1 (or even more).
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hisFeb 28 '14 at 19:34

@his: Most cameras don't really work on 18% gray. They often look for bright areas (more than just a reflection highlight) and expose for them near the top. However, the problem with snow is that if a normal brightness subject is in the center and large enough, some metering systems will ignore the bright snow on the edges and blow it out.
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Olin LathropFeb 28 '14 at 20:09

I highly doubt that claim. Most metering (matrix, weighted) will take the snow into account and will underexposure. And a little googling shows me that this is not only my personal experience but seems to be the overwhelmingly most common hint: google.com/#q=exposure+compensation+snow If you are using spot metering you practically do it manually. Of course you need to know what to do then.
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hisMar 1 '14 at 10:08